One of the problems associated with air jets of this type is that of threading the yarn or yarns through the yarn channel in the jet body. The original methods of threading using a needle or weight attached to the yarn were very time consuming and are not acceptable for modem high speed yarn processing machines. There are many jet designs which incorporate a threading slot communicating with the yarn channel over the length of the jet body, but such slots can impair the air flow in the yarn channel, create a tendency for the yarns to migrate into the slot, thereby preventing correct processing of the yarn, or even allow the yarn to escape from the jet. To avoid such problems, various arrangements have been devised for opening the jet to expose the yarn channel for access from the outside of the jet for yarn threading purposes and then for closing the jet to commence yarn processing. In one known type of jet, the yarn channel is in a central part of the jet body which is arranged to slide laterally relative to upper and lower parts, thereby exposing the channel and simultaneously closing the air inlet to the sliding central part to stop the flow of air into the yarn channel. This involves one surface sliding under pressure across a seal, which leads to rapid wear of the seal. Furthermore, the pivoted lever mechanism used to produce the sliding motion puts considerable stresses on the parts and in consequence is also prone to wear. Therefore, such an arrangement leads to high maintenance costs. In addition, this type of jet is very bulky and space for air jets in textile machines is restricted. Another type of opening air jet involves an upper part of the jet rotating relative to the lower part to expose the yarn channel in the lower part and simultaneously stop the flow of air through the lower part to the yarn channel. Such an arrangement has the serious disadvantage that the air inlet must be offset from the yarn channel in order that it can be closed by the rotated upper part when the channel is exposed for threading purposes, and in consequence during yarn processing the path of air from the inlet through the two parts of the jet to the yarn channel involves several changes of direction. This seriously reduces the air flow and its pressure when it reaches the yarn channel, thereby reducing the processing effectiveness of the air jet. Another problem is that exposing the yarn channel before the air is switched off and switching the air on whilst the yarn channel is still exposed can cause the yarn or at least some filaments to be blown out of the channel with the possibility of snagging on adjacent machine parts. In addition, maintaining satisfactory sealing of the resulting tortuous yam path through the two parts of the jet is difficult.